1. Field of the Invention
The present invention relates to a current source and a current supply system, and more particularly, to a current source and a current supply system with hysteresis.
2. Description of the Prior Art
In a common low dropout (LDO) regulator, a control circuit is disposed to maintain the LDO's output voltage when a load arrives. For the control circuit, it is desirable to have a rapid response and low power consumption. However, there is a tradeoff between the response speed and power consumption. With a higher bias current, the control circuit may respond faster but consume more power. With a lower bias current, the control circuit may have less power consumption, which is accompanied by lower response speed.
An adaptive bias current source is therefore applied to achieve high response speed and low power consumption simultaneously. The adaptive bias current source may detect the status of an output circuit and supply a larger current only when the output circuit requires a faster response, e.g., when a load variation occurs in an LDO regulator. In other words, the adaptive bias current source is capable of detecting the status of the output circuit and accordingly supplying adaptive bias currents to the output circuit such as an LDO regulator, an amplifier or a buffer. However, if the adaptive bias current source is excessively sensitive to the status of the output circuit, the output circuit may become unstable since the current supplied by the adaptive bias current source always varies. Thus, the adaptive bias current source is always implemented with hysteresis characteristics, to avoid the stability problem.
Please refer to FIG. 1, which is a schematic diagram of a common current source 10 with hysteresis. The current source 10 includes a tail current transistor mbn1 and a differential pair, which includes transistors mn1 and mn2. The tail current transistor mbn1 receives a bias voltage VBN to generate a tail current I1 to be supplied to an output circuit. The transistors mn1 and mn2 respectively receive an output voltage VOUT and an input voltage VIN of the output circuit, to detect the status of the output circuit. Hysteresis is achieved by applying different width/length ratios (W/L) in the transistors mn1 and mn2, as denoted by:(W/L)1=α(W/L)2.  (1)wherein α is a positive constant other than 1, and (W/L)1 and (W/L)2 are width/length ratios of the transistors mn1 and mn2, respectively.
The hysteresis effect of the current source 10 may be determined by the drain current formula of a metal oxide semiconductor field-effect transistor (MOSFET), i.e., ID=K′(W/L)×(Vgs−Vth)2, which is described as follows:ID1=K′(W/L)1×Vov12; ID2=K′(W/L)2×Vov22;  (2)wherein ID1 and ID2 are drain currents of the transistors mn1 and mn2, respectively, K′ is a constant, and Vov1 and Vov2, which equal to the gate-source voltage (Vgs) minus the threshold voltage (Vth), are overdrive voltages of the transistors mn1 and mn2, respectively. When |VOUT−VIN|=0 and Vov1=Vov2, Equation (2) is substituted by Equation (1) to obtain:ID1=αID2.  (3)
According to the above equations, the transition point of the voltages |VOUT−VIN| may be calculated to be equal to |VOUT−VIN|=[(1+α)1/2−1] Vov. As can be seen, the hysteresis effect in the current source 10 is determined by the value of the overdrive voltage of the transistors mn1 and mn2 in the differential pair. In general, considering the static power consumption, the current source 10 is preferably designed to possess a low static current. Therefore, the transistors mn1 and mn2 may have low overdrive voltages, e.g., 20 mV, or may even be operated in the sub-threshold region. In such a condition, since the overdrive voltages of the transistors mn1 and mn2 are quite small, the current source 10 may have a poor hysteresis effect, and the adaptive range of hysteresis is also small.
The output circuit, such as the LDO regulator or the buffer, implemented with the current source 10 may easily become unstable due to the poor hysteresis effect of the current source. Thus, there is a need for improvement over the prior art.